Today, an increasing number of illnesses and dysfunctions are corrected or controlled by the implantation into the body of devices. Examples are the common pacemaker, the anti-tachycardia pacemaker and the automatic implantable defibrillator, which are designed to correct irregularities such as tachycardia, or ventricular or auricular fibrillation. Other devices are currently under research or development, or will be in the future, such as drug infusion pumps, pain relief stimulators or implantable artificial hearts and kidneys.
Very significant progress has been made in the functionality of implantable devices, along with size reduction and increase in longevity, as a result of advances in batteries, circuits and technology.
Basically, an implantable device is composed of a) an actuator, performing the physical function, and b) a control electronics of the actuator as well as of the other auxiliary elements all requiring energy (generally electric) supplied by a battery.
Nickel-cadmium and zinc-mercury batteries were used to supply the first generations of implantable devices, but their large size and short life span made nuclear batteries a potential option soon disregarded due to the very strict regulations applying to them. Since then, lithium batteries have become those most used because of their longer duration, comparable to the nuclear cells but subject to less stringent regulations. Additionally, lithium batteries offered several advantages such as a discharge indicator, no internal current leakage (self-discharge), and lack of gaseous emissions. This provides conditions for more reliable performance.
Even if the improvements in longevity are taken into account, the concept of battery is still that of a receptacle containing a limited amount of energy which runs out after a certain amount of time has elapsed. In the case of implantable medical devices this fact leads to three situations:
a) Replacement of the exhausted batteries (more precisely, replacement of the whole implantable device which is hermetically sealed), with a new set. This replacement is carried out by means of surgical intervention, which in some cases exposes the patients to some risks, as well as substantial costs to the health-care system, contrary to the worldwide trend of reducing medical costs. PA1 b) Limitation on the potential for today's implantable devices. In fact, the evolution of the technology in implantable medical devices has drastically reduced the energy requirements of the actuator, making the enhancement and extension of the device's functionality for a given battery capacity possible. Among the possible uses for the energy saved from more efficient actuators, trends point towards its use in increased automation, and the incorporation of distant diagnosis and monitoring by telemetry. As an example, in today's common pacemakers, 50% to 65% of the energy is devoted to these additional functions with predictions of a rise to 95% in the near future. PA1 c) Stagnation in new developments in medical implantable devices, requiring a much larger amount of energy than devices available today. The only possible way with present technologies, is to oversize the batteries (in energy, and consequently in physical dimensions), or to reduce the usable time. This is the case in the implantable defibrillator, whose operational principle requires a high voltage, drawing a large amount of energy every time it is actuated. The typical lifetime of these devices is 2 to 3 years. Other examples such as the artificial implantable heart requiring an extremely large amount of energy, are not viable using present technologies and batteries. PA1 permanent performance, PA1 availability of energy levels high enough to supply devices with increased functionality, or future devices requiring a much larger amount of energy, and PA1 miniaturization of the system due to large batteries not being required to keep the device operational for many years. PA1 potential rejection from the patient's body of the optical fiber placed permanently inside, and PA1 potential infection because one of the ends of the optical fiber arrives to the patient's surface thus becoming a potential entry point for infection. PA1 Optical fiber is a biocompatible material, made of inert plastics (of medical grade) which have already been successfully used in other medical applications, and PA1 Optical fibers have very small diameters, several tens of microns in diameter, (similar to that of a human hair). Therefore, the end of the fiber close to the skin would be sealed by natural means (epithelialization). This is a situation already solved in other medical applications, such as internal hypodermic reservoirs and pumps for analgesic drugs.
However, if the energy saved from enhanced efficiency of the actuator is to be used in increased functionality, other not less attractive possibilities, such as the increase in the battery lifetime, or the reduction in the battery size, may be out of scope (as the battery takes up the biggest part of the whole device, if battery size were reduced the size of the whole device would be miniaturized as well). Therefore, a trade-off appears between the different options available for using the extra energy saved by the actuators, but not all of these options can be considered simultaneously.